agent: improve warning message in case of error after TriggerStop
agent: improve warning message in case of error after TriggerStop

Improve warning message from TriggerStop() in case of error,
because it's just a trigger, not the actual stopping.

Suggested by @jonboulle

dongsupark/fleet,giantswarm/fleet,dalbani/fleet,endocode/fleet,jonboulle/fleet,endocode/fleet,coreos/fleet,endocode/fleet,dalbani/fleet,giantswarm/fleet,dongsupark/fleet,dalbani/fleet,coreos/fleet,giantswarm/fleet,coreos/fleet,dongsupark/fleet,jonboulle/fleet,jonboulle/fleet